Radiation-curable composition

ABSTRACT

A radiation-curable composition for use as a base coat in printing on plastic substrates such as polycarbonate. The radiation-curable composition comprises: (i) an ethylenically unsaturated monomer that is capable of being photopolymerized; (ii) a photoinitiator that is capable of initiating the polymerization of the ethylenically unsaturated monomer (i) when irradiated with ultra-voilet light; (iii) an inorganic transfer agent; (iv) an adhesion promoter; and (v) optionally, a pigment. The base coat is suitable for use in printing plastics substrates using electrostatic offset printing.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority of Great Britain No. PCT/GB00/04798filed Dec. 20, 1999, the complete disclosure of which is herebyincorporated by reference.

This invention concerns a radiation-curable composition. In particular,this invention concerns a radiation-curable composition that can be usedas a base coat for, for example, electrostatic offset inks.

In electrostatic offset printing an image is formed by directing one ormore pigmented inks or toners on to a selectively charged drum, whichare then transferred, either directly or via an intermediate member, onto a substrate. Electrostatic offset printers are made and sold by, forexample, Indigo NV and by Xeikon SA.

Inks used in offset printing have difficulty adhering to certainsubstrates such as plastics. This can be overcome by modifying thesurface of the substrate. The surface can be modified by the applicationof a preliminary base coat comprising, for example, a solvent-basedresin such as, for example, a polyamide, a butadiene resin or anethyleneacrylic acid resin (see JP 10076744). When a solvent-based resinis used, the solvent needs to be evaporated before printing, which canbe slow, hazardous and offensive, and is environmentally unfriendly.

WO 98/49604 discloses a copolymer receptor layer which is applied byextrusion to a polyvinyl chloride backing to improve abrasion resistanceof a printed image. The copolymer receptor layer can be used to improveprint quality on a polycarbonate backing layer. In both of theseapplications, we are told that the copolymer receptor layer adheres tothe polyvinyl chloride backing layer in the absence of UV radiation.

The aim of the present invention is to provide a base coat that can beused on difficult substrates, such as plastic substrates, that areprinted using, for example, offset printing.

A further aim of the present invention is to provide a base coat thatdoes not contain volatile organic solvents.

A further aim of the present invention is to provide a base coat thatcan be applied by screen process printing or by any other suitablecoating or printing process.

In accordance with the present invention there is provided aradiation-curable composition for use as a primer base coat forelectrostatic offset printing, the composition comprising:

(i) an ethylenically unsaturated monomer, oligomer or polymer that iscapable of being photopolymerized;

(ii) a photoinitiator that is capable of initiating the polymerizationof the ethylenically unsaturated monomer (i) when irradiated withultra-violet light;

(iii) optionally, a pigment;

(iv) an inorganic transfer agent; and

(v) an adhesion promoter.

In accordance with the present invention there is also provided a methodfor printing a substrate, the method comprising a stop of applying abase coat to the substrate before printing, the base coat comprising thecomposition defined above.

In accordance with the present invention there is also provided use ofthe composition defined above as a base coat for printing.

The base coat may be applied to a substrate using, for example, silk orscreen process printing, flexographic printing, curtain coating, rollercoating and spin coating.

In use, the base coat is applied to a substrate and then irradiatedusing ultra-violet light. The cured base coat possess excellentproperties for the reception of images produced by electrostatic offsetprinting. The base coat can be used, for example, for printing on tocompact discs, labels or similar plastics articles.

The monomer, oligomer or prepolymer (I) is preferably an ester ofacrylic or methaorylic acid, such as, for example, phenoxyethylacrylate, hexanediol diactylate, tri(propylene glycol) diacrylate,trimethylolpropane triacrylate, polyethylene glycol diacrylatehydroxyathyl methaerylate, trirnethylolpropane triacrylate,tetrahydrofurfuryl acrylate, lauryl acrylate, octyl acrylate, decylacrylate, acrylates of polyurethane, polyester, polyether, melamine orepoxy resins, ethoxylated or propoxylated derivatives of any of theaforementioned acylates or methacrylates; or a N-vinylamide such as, forexample, N-vinylcaprolaotam or N-vinylformamide; or a mixture thereof.The monomer, oligomer or prepolymer is preferably present in an amountfrom 10 to 80%, more preferably 40-60%, by weight of the composition.

The photoinitiator (ii) is of the typo known to produce free radicalswhen irradiated. The photoinitiator is, for example, benzophenone.1-hydroxycyclohexyl phenyl ketone,2-benzyl-2-dimethylamino-(4-morpholinaphenyl)butan-1-one, benzildimethylketal, isopropyltnoxanthone,bis(2,6-dimethylbenzoyl)-2,4,4-trimethylpentylphosphine oxide or amixture thereof.

Colourless or nearly colourless materials are preferred; however, yellowcoloured photoinitiators may also be used. The photoinitator ispreferably present in an amount from 0.5 to 20%, more preferably from2.5 to 8%, by weight of the composition.

The inorganic transfer agent is required to ensure good transfer ofelectroink from the electrostatic offset press blanket to the curedbasecoat. The inorganic transfer agent (iv) is preferably a silica or amodified silica. Suitable silicas are Aerosil 200 (available fromDegussa) and Cab-o-sil TS610 (available from Cabot), or a mixturethereof. It is particularly advantageous to use commercial materials inwhich the silica or modified silica is dispersed in an ethylenicallyunsaturated monomer ingredient of type (i), such as, for example.Highlink OG (available from Clariant). We have found that these transferagents, especially silicas, render the final cured ease coatparticularly receptive to electrostatic offset ink. They also have theadvantage of giving a desirable ‘structure’ to the compositions, i.e.resistance to flow under gravity without excessive viscosity at highershear. The inorganic transfer agent (iv) is preferably present in anamount from 0.5 to 50%, more preferably from 4 to 35%, by weight of thecomposition.

The adhesion promoter (v) is present to aid adhesion of theelectrostation offset ink to the cured base coat. Suitable materials arepolar organic nitrogen compounds having a molecular weight preferablygreater than 200 and preferably containing two or more nitrogen atomssuch as, for example, the amines sold by Huntsman Incorporated under thename Jeffamine D230 (a diamine); and Jeffamine T403 (a triamine); orpolyamides, such as, for example, Solsperse 32000 (available fromZenoca), described as a polymer acid salt/polymer amide; or a mixturethereof. It is known in the art that amines may be used to act assynergists for photoinitiators; however, in this invention, the aminesare added to aid the adhesion of the polar electrostatic offset ink tothe base coat. The adhesion promoter is preferably present in an amountfrom 5 to 25%, more preferably from 11 to 19%, by weight of thecomposition.

Optionally, the following additives may also be present: passive resins(i.e. resins that are not cured by ultra-violet light) to stabilise theviscosity of the composition and improve the physical properties of thecured base coat; dyes or pigments, especially white pigment to form anopaque background for the electrostatic offset print; stabilisers toprevent deterioration of the composition to heat or on storage; flowaids; biocides; defoamers; and surfactants.

The components of the base coat are preferably selected so that they arecompatible with each other and provide a mixture stable on storage, withthe required rheological and other properties. In particular, thecompositions can preferably be formulated to give a rheological profilesuitable for application by screen-printing. For this purpose their gelstrength, as measured as the viscosity at 25° C. on a Brookfield RVDTdigital viscomoter at Sr.p.m using a No.6 spindle, preferably liesbetween 18 and 1500 poise, more preferably between 100 and 600 poise,and their viscosity under higher shear, as measured on the Rotothinner(supplied by Sheen Instruments), is preferably between 3 and 65 poise,more preferably 20 and 50 poise. Alternatively, the compositions can bestared and supplied in two parts: the adhesion promoter being in onepart and the rest of the composition being in the other part.

The substrate may be polycarbonate, which is used in the production ofcompact discs or other plastics materials such as, for example,poly(vlnyl chloride), toils or papers. Preferred substrates are plasticsarticles such as labels and overlay film, especially uncoated andspin-coated polycarbonate compact discs or DVD's.

The substrate may be printed using a screen stencil or by any otherappropriate printing method, and then hardened by exposure toultra-violet light or cold-cure systems. Typical exposures are at aspeed of 27-50 meters/minute on a Svecia u.v. dryer having two mediumpressure mercury lamps operating at 80 watts/centimetre, or at up to 56impressions per minute with two lamps at 100% power on a Kammann K15printer. After curing, the substrate bearing the base coat is especiallysuitable for receiving electrostatic offset ink.

The invention will now be described, by way of example, with referenceto the following Examples (all pans are by weight):

EXAMPLE 1

The following components were mixed on a Torrance stirrer fitted with awater-cooling jacket:

Hexane diacrylate 76.8 parts Jeffamine D-230 12.5 parts1-Hydroxycyclohexyl phenyl ketone 2.5 parts Aerosil 200 3.1 partsAerosil R972 3.1 parts

The first two components were mixed at 50° C. to give a homogeneoussolution, which was cooled for the later additions. Finally after theaddition of the Aerosil 200, the speed of the stirrer was increased andthe mixture was allowed to heat to 50° C. until no nibs were detectedusing a Hegman gauge.

The composition had a Rotothinner viscosity of 25 poise at 25° C. andBrookfield viscosity of 45 poise at 25° C.

The composition was printed on a Kammann K15H printing machine at 56impressions per minute through a screen stenoil prepared on a mashhaving ISO threads per centimetre on to a blank polycarbonate compactdisc, and cured with two u.v. lamps at 100% power. The resulting coateddisc was an excellent substrate for an electrostatic offset ink appliedby an Indigo printer.

The quality of the image printed by the Indigo printer on the coateddisc was superior to one applied by screen process printing and wascomparable to a lithographic print.

EXAMPLE 2

The following components were mixed using a Grieves stirrer.

Highlink OG 103.53 80.0 parts Jeffamine T403 12.2 parts1-Hydroxycyclohexyl phenyl ketone 3.7 parts Aerosil 200 3.3 parts

The composition had a Rotothinner viscosity of 21 poise at 25° C., and aBrookfield viscosity of 510 poise at 25° C.

The composition was prepared in the same way as in Example 1.

EXAMPLE 3

The following composition was prepared:

Jeffamine D-230 10.0 parts Hexanediol diacrylate 73.0 parts FlorstabUV-1 stabiliser from Kromachem Ltd 1.0 parts 1-Hydroxycyclohexyl phenylketone 4.5 parts Adhesion resin LTH from Huels AG 5.5 parts Aerosil 2003.0 parts Aerosil R972 3.0 parts

The composition had a Rotothinner viscosity of 3 poise at 25° C. and aBrookfield viscosity 0 of 10 poise at 25° C.

The composition was prepared by mixing the components in the order theyare given on a Torrance stirrer fitted with a water-cooling jacket.Finally the mixture was milled on a Buhler triple-roll mill until nonibs were visible on a Hegman gauge.

This composition can be provided as a two part pack with the JeffamineD230 being in one pack and the other components being in the other pack.The Jeffamine should be combined with the other components before use.

EXAMPLE 4

The following composition was prepared:

Highlink OG 103-53 75.3 parts Jeffamine diamine D-230 9.3 parts1-Hydroxycyclohexyl phenyl ketone 3.7 parts Ebecryl B70 (acrylate resinfrom UCB) 6.0 parts Aerosil 200 3.0 parts Hexanediol diacrylate 2.7parts

The composition (before addition of the diamine) had a Rotothinnerviscosity of 32 poise at 25° C. and a Brookfield viscosity of 560 poiseat 25° C.

The components were mixed in the order given on a Torrance stirrerfitted with a water-cooling Jacket. After the last addition the speed ofthe stirrer was increased and the mixture was allowed to heat to 50° C.until no nibs were visible on a Hagman gauge.

The following composition was prepared:

Highlink OG 103-53 61.8 parts Diamine D-230 10.0 parts1-Hydroxycyclohexyl phenyl ketone 3.0 parts Lucerin TPO 2.0 partsAerosil 200 0.7 parts Finntitan RDDI pigment 22.0 parts

The composition was prepared as in Example 1. The composition had arotothinner viscosity of 22 poise at 25° C. and a Brookfield viscosityof 18 poise at 25° C.

What is claimed is:
 1. A radiation-curable composition for use as aprimer base coat for electrostatic offset printing, the compositioncomprising: (i) an ethylenically unsaturated monomer, oligomer orpolymer that is capable of being photopolymerized; (ii) a photoinitiatorthat is capable of initiating the polymerization of the ethylenicallyunsaturated monomer (i) when irradiated with ultraviolet light; and(iii) an inorganic transfer agent; and (iv) an adhesion promoter.
 2. Theradiation-curable composition of claim 1, wherein the monomer, oligomeror polymer is selected from the group consisting of esters of acrylicand methacrylic acid and N-vinylamide and mixtures thereof.
 3. Theradiation-curable composition of claim 2, wherein the ester isphenoxyethyl acrylate, hexanediol diacrylate, tri(propylene glycol)diacrylate, trimethylolpropane triacrylate, polyethylene glycoldiacrylate, hydroxyethyl methacrylate, trimethylolpropane triacrylate,tetrahydrofurfuryl acrylate, lauryl acrylate, octyl acrylate decylacrylate, acrylates of polyurethane, polyester, polyether, melamine orepoxy resins, ethoxylated or propoxylated derivatives of any of theaforementioned acylates or methacrylates or a mixture thereof.
 4. Theradiation-curable composition of claim 2 wherein the N-vinylamide isN-vinylcaprolactam or N-vinylformamide.
 5. The radiation-curablecomposition of claim 1, wherein the monomer, oligomer or polymer ispresent in an amount from 10 to 80%, by weight of the composition. 6.The radiation-curable composition of claim 1, wherein the photoinitiatoris benzophenone, 1-hydroxycyclohexyl phenyl ketone,2-benzyl-2-dimethylamino-(4-morpholinophenyl)butan-1-one, benzildimethylketal, isopropylthoxanthone,bis(2,6-dimethylbenzoyl)-2,4,4-trimethylpentyl-phosphine oxide or amixture thereof.
 7. The radiation-curable composition of claim 6,wherein the photoinitiator is colourless or substantially colourless. 8.The radiation-curable composition of claim 1, wherein the photoinitiatoris present in an amount from 0.5 to 20%, by weight of the composition.9. The radiation-curable composition of claim 1, wherein the inorganictransfer agent is a silica or modified silica.
 10. The radiation-curablecomposition of claimed in claim 9, wherein the silica or modified silicais dispersed in the ethylenically unsaturated monomer.
 11. Theradiation-curable composition of claim 1, wherein the inorganic transferagent is present in an amount from 0.5 to 50%, by weight of thecomposition.
 12. The radiation-curable composition of claim 1, whereinthe adhesion promoter is a polar organic nitrogen compound.
 13. Theradiation-curable composition of claim 12, wherein the adhesion promoteris a diamine, a triamine, or a mixture thereof.
 14. Theradiation-curable composition of claim 12, wherein the adhesion promoteris present in an amount from 5 to 25% by weight of the composition. 15.A method for printing a substrate comprising applying a primer base coatof the composition of claim 1 to the substrate before printing.
 16. Themethod of claim 15, wherein the substrate is polycarbonate.
 17. Themethod of claim 15 wherein the monomer, oligomer or polymer is selectedfrom the group consisting of esters of acrylic and methacrylic acid andN-vinylamide and mixtures thereof.
 18. The method of claim 15 whereinthe ester is phenoxyethyl acrylate, hexanediol diacrylate, tri(propyleneglycol) diacrylate, trimethylolpropane triacrylate, polyethylene glycoldiacrylate, hydroxyethyl methacrylate, trimethylolpropane triacrylate,tetrahydrofurfuryl acrylate, lauryl acrylate, octyl acrylate decylacrylate, acrylates of polyurethane, polyester, polyether, melamine orepoxy resins, ethoxylated or propoxylated derivatives of any of theaforementioned acylates or methacrylates or a mixture thereof.
 19. Themethod of claim 15 wherein the N-vinylamide is N-vinylcaprolactam orN-vinylformamide.
 20. The composition of claim 1 which further comprisesa pigment.